Since the update my iPhone stops tracking after I put it in my pocket?

If you started to see this happen after you updated your iOS software chances are you need to enableBackground App Refresh.

Apple implemented this feature allowing you to disable individual applications from running in the background. Turning off apps may help preserve the battery life; however, it will prevent the software from continuing to track you.

Go into your main iPhone Settings and select General, followed by Background App Refresh -> Verify you have Background App Refresh enabled (green), along with the individual application.